igneous
Weathered basalt or iron-rich river cobble
Basalt, likely with iron-oxide staining; exact identification requires testing
AI-generated identification · may be incorrectDark gray-black, fine-grained, rounded and possibly water-worn, with rusty orange-brown iron-oxide patches. Basalt is typically hard (Mohs 6), dull to subvitreous, dense, and lacks visible cleavage; the orange areas may be limonite/goethite.
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Physical properties
Dark gray-black, fine-grained, rounded and possibly water-worn, with rusty orange-brown iron-oxide patches. Basalt is typically hard (Mohs 6), dull to subvitreous, dense, and lacks visible cleavage; the orange areas may be limonite/goethite.
Formation & geological history
Likely formed as an ancient volcanic lava flow, then weathered and rounded in a stream or shoreline environment; basalt commonly ranges from Archean to Recent in age. An alternative is an iron-rich metamorphic or industrial slag fragment, which cannot be excluded from the image alone.
Uses & applications
Common basalt is used as aggregate, road stone, landscaping rock, and occasionally building stone; it has little gem value. Attractive polished cobbles may be sold as low-cost decorative specimens.
Geological facts
The specimen’s smooth oval shape suggests transport by moving water or wave action. A strong magnet, if present, could indicate magnetite-rich basalt or slag, while abundant vesicles or glassy texture would favor volcanic rock or slag respectively.
Field identification & locations
Look for tiny interlocking crystals, high density, and a streak that is not strongly metallic; basalt usually does not fizz in dilute hydrochloric acid. Likely locations include volcanic terrains, gravel bars, beaches, and glacial deposits.
